Direct Evidence of Abrupt Changes in Two-Dimensional Fluidity in Melting of Monolayers

Debye-Waller factors exp(-2W) of monolayer films of butadiene iron tricarbonyl molecules adsorbed on basal-plane graphite surfaces have been measured in the surface-normal and surface-parallel directions. For each coverage x=0.3, 0.7, and 1.0, the normal exponent 2W_⊥ indicates strong substrate binding at all temperatures, while the parallel one 2W_∥ increases abruptly at a definite temperature, indicating a first-order melting transition in the plane of the film.